The present invention relates to a thermo-actuator, and more particularly to a thermo-actuator provided in a wax-pellet thermostat for a cooling system of an automotive engine and for various thermo-sensors.
The thermo-actuator in the wax-pellet thermostat includes a steel rod secured to a housing at an end thereof, a guide member slidably mounted on the rod, a seal spool secured to the guide member and provided around the rod, a heat conductive cylinder secured to the guide member and exposed to a coolant, a wax provided in the cylinder, and a spring urging the guide member towards the end of the rod.
When the temperature of the coolant rises in excess of a predetermined value of the thermostat, the wax expands. This forces the seal spool against the steel rod. As the wax squeezes the seal spool around the rod, the cylinder moves outwardly together with the guide member against the spring to open a main valve to pass the coolant to a radiator, thereby lowering the temperature of the coolant. When the thermostat cools, the wax contracts, so that the seal spool expands, and the cylinder is moved to the end of the rod by the spring to close the valve. In order to seal a sliding portion of the guide member, a dynamic seal such as a packing is provided. However, the coolant includes fine molding sands, fine particles of rusted iron, oil and other particles which form a sludge with antifreeze mixture. Therefore, the conventional seal is liable to be deteriorated because of severe circumstances.
On the other hand, in an ordinary thermostat having a valve open lift of 10 mm, the pressure of the wax for lifting the valve becomes high as 140 kg/cm.sup.2. To resist the high pressure and to ensure the durability, the seal spool is made to have a large thickness, for example 45% of the diameter of the rod. Therefore, in order to compress and deform the thicken and stiff seal spool, a large force of 38 kg/cm.sup.2 for 140 kg/cm.sup.2 is consumed. In addition, there probably occur troubles, such as wear and tear of the seal spool, and sticking of the spool to the rod, because of large friction.